I mostly want to comment on some assumptions across posts (stated directly or implied) that I think are more up for interpretation or that may neglect possibilities.
First, Alan having a smaller penis relative to some others doesn't necessarily mean that he cannot satisfy his wife adequately over time (at least enough for her under normal circumstances), and it also does not mean that Alan would not be able to sexually satisfy Anna. Alan does not have a micro-penis, and different people have various preferences and weight the aspects of a sexual experience differently, with the act of penetration (and specifically how large of an object is involved in penetration) being of more or less importance across individuals. Larger penis size may not factor into Anna's sexual pleasure as much as it currently seems to for Clara, and case in point lesbian couples can give each other plenty of orgasms, with more emphasis on clitoral stimulation (whether without penetration or in leading up to or accompanying penetration). Also, Alan possibly could become more sexually adventurous/passionate over time within his marriage, and his sex/love making style may be more satisfying for Anna (e.g., gentler/more tender vs. rougher/more dominating).
Second, I do not see Alan as being a loser. Yes, he has a thinner frame, is not muscular or well-endowed, and is not sexually confident/is less sexually experienced than others. For example, Clara thinks to herself multiple times that what she's doing toward Alan at X's direction was the first time he had experienced something like that, and in regard to expecting him to cum within 60 seconds, she admits it is a sudden and unfair expectation. However, I would not say that this makes Alan a loser. I give him credit for supporting his wife as best he can when possible (at least on one path), and looking beyond sex, Alan does have positive qualities that other men lack. Having said that, I do hope that Alan (at least on one of the paths) becomes bolder and more assertive. Hopefully, the "Boldness" points that can be awarded, such as when Alan chooses to tell Clara off, will come into play more later in the game and will have an impact on character/relationship development.
Lastly, Clara having cheated with Leon was definitely a mistake, especially given the history between Alan and Leon. However, without excusing her actions (which even the character herself doesn't care to do), there were unusual circumstances. She was manipulated at X's direction into thinking that Alan was cheating with Anna, and then Leon just happens to pop by when she's intoxicated and vulnerable and pressures her to respond sexually amidst her protesting multiple times. Yes, she ultimately succumbed to his advances, but we all know that Leon's story about "please excuse my behavior but I was so concerned about you and don't know what came over me" is a line of bullshit. Thus, while I think Clara messed up, I don't think she's beyond redemption or that her marriage is unsalvageable. Hopefully, there will be a path where Clara and Alan wise up about being manipulated and more openly communicate with each other, to where they'll be more understanding of their actions (if not excusing then able to forgive) and again establish more closeness.